A backslash followed by a space inside a link destination is a literal
backslash (a space is not ASCII punctuation, so it is not escaped), and
the space ends the destination. markdown-it instead broke at the
backslash, dropping it from the destination and failing to parse the
link.
Repro:
md.render('[a](/url\\ )')
Expected (matches the CommonMark 0.31.2 reference renderer):
<p><a href="/url%5C">a</a></p>
Actual on master:
<p>[a](/url\ )</p>
In lib/helpers/parse_link_destination.mjs the unenclosed-destination
loop, on seeing `\` followed by a space, called `break`, which excludes
the backslash from the destination and leaves `\ )` unconsumed so the
link fails. Advance past the backslash instead (`pos++; continue`) so it
is kept literal and the following space triggers the normal terminator.
The existing commonmark/CommonMark#493 case `[a](a\ b)` is unaffected:
content after the space still prevents the link from parsing, matching
the reference.
A backslash followed by a space was baking that space into the escape
token, so the newline rule saw only one trailing space and emitted a
soft break instead of the mandated hard line break.
Per CommonMark 0.31.2 section 6.1, a backslash before a space is a
literal backslash and the space is not consumed. Leaving the space in
the text stream lets section 6.7 two-space hard-break detection fire.
Input `a\ \nb` (backslash + two trailing spaces) now renders
`<p>a\<br>\nb</p>` instead of `<p>a\ \nb</p>`.
Vitaly Puzrin